Reimplement and document ldict-filter.

doc/utils.md

@@ -112,6 +112,15 @@ if it accepts three arguments, the key, value and numeric index
 In all cases the value the procedure returns is taken as the new value
 for given key.
 
+    (ldict-filter pred? ld)
+
+* ```pred?``` - predicate procedure
+* ```ld``` -a ldict instance
+
+Returns a new dictionary containing only key/value pairs matching the
+given predicate. The procedure ```pred?``` must accept two arguments -
+the key and the value.
+
 ### IO
 
     (import util-io)

src/util-dict.scm

@@ -179,15 +179,17 @@
  ;; Returns a dictionary containing only kv pairs matching the
  ;; predicate which must accept two arguments. Unlike list filter,
  ;; does not perform final reverse on the result.
- (define (dict-filter pred? d)
-   (let loop ((d d)
-	      (r '()))
-     (if (null? d)
-	 r
-	 (loop (cdr d)
-	       (if (pred? (caar d) (cdar d))
-		   (cons (car d) r)
-		   r)))))
+ (define (ldict-filter pred? ld)
+   (let loop ((pairs (ldict-pairs ld))
+	      (res '()))
+     (if (null? pairs)
+	 (cons TAG-LDICT
+	       (cons (ldict-meta ld)
+		     res))
+	 (loop (cdr pairs)
+	       (if (pred? (caar pairs) (cdar pairs))
+		   (cons (car pairs) res)
+		   res)))))
 
  ;; Reduce over dictinary, the reducing procedure gets accumulator,
  ;; key and value as its three arguments.